Field of the Technology
The present disclosure is directed to thread rolling dies used for producing threads on one machine component in order to fasten it to another machine component, and to methods of manufacturing thread rolling dies. More specifically, the disclosure is directed to thread rolling dies comprising sintered cemented carbide thread rolling regions, and to methods of making the thread rolling dies.
Description of the Background of the Technology
Threads are commonly used as a means of fastening one machine component to another. Machining techniques such as turning, using single point or form tools, and grinding, using single contact or form wheels, are employed as metal removal methods to create the desired thread geometry in a workpiece. These methods are commonly referred to as thread cutting methods.
Thread cutting techniques suffer from some inherent disadvantages. Thread cutting techniques are generally slow and costly, and require the use of expensive machine tools, including special tooling. The thread cutting techniques are not cost-effective for processing large production batches. Because thread cutting involves machining a blank, waste material in the form of cut chips is produced. Additionally, the finish of cut threads may be less than desirable.
An alternative method of forming threads in machine components involves the use of “chipless” metal forming techniques, i.e., thread forming techniques in which the workpiece is not cut and chips are not formed. An example of a chipless thread forming technique is the thread rolling technique. The thread rolling technique involves rolling threads onto a cylindrical metal component positioned between two or more thread rolling dies including a working surface having a mirror-image of the desired thread geometry. Traditionally, thread rolling dies may be circular or flat. The thread geometry is created on a workpiece as it is compressed between the dies and the dies move relative to one another. Circular thread rolling dies are rotated relative to one another. Flat thread rolling dies are moved in a linear or reciprocating fashion relative to one another. Thread rolling is therefore a method of cold forming, or moving rather than removing the workpiece material to form the threads. This is illustrated schematically in FIGS. 1A and 1B. FIG. 1A schematically illustrates a thread rolling die positioned on a side surface of a cylindrical blank, and FIG. 1(b) schematically illustrates the final product produced by rotating the blank relative to the die. As indicated in FIGS. 1A and 1B, the process of moving the material of the blank upward and outward to form the threads results in a major thread diameter (FIG. 1A) that is greater than the blank diameter (FIG. 1B).
Thread rolling offers several advantages over machining or cutting techniques for forming threads on a workpiece. For example, a significant amount of material may be saved from becoming waste because of the “chipless” nature of the thread rolling technique. Also, because thread rolling forms the threads by flowing the material upward and outward, the blank may be smaller than that required for when forming the threads by thread cutting, resulting in additional material savings. In addition, thread rolling can produce threads and related forms at high threading speeds and with longer comparable tool life. Therefore, thread rolling is a viable technique for high volume production. Thread rolling also is cold forming technique in which there is no abrasive wear, and the thread rolling dies can operate throughout their useful life without the need for periodic sizing.
Thread rolling also results in a significant increase in the hardness and yield strength of the material in the thread region of the workpiece due to work hardening caused by the compressive forces exerted during the thread rolling operation. Thread rolling can produce threads that are, for example, up to 20% stronger than cut threads. Rolled threads also exhibit reduced notch sensitivity and improved fatigue resistance. Thread rolling, which is a cold forming technique, also typically results in threads having excellent microstructure, a smooth mirror surface finish, and improved grain structure for higher strength.
Advantages of thread rolling over thread cutting are illustrated schematically in FIGS. 2A and 2B. FIG. 2A schematically shows microstructural flow lines in a thread region of a workpiece resulting from thread cutting. FIG. 2B schematically shows microstructural flow lines in a thread region of a workpiece resulting from thread rolling. The figures suggest that no material waste is produced by thread rolling, which relies on movement of the workpiece material to produce the threads. The flow lines shown in FIG. 2B also suggest the hardness improvement and strength increase produced by flowing of material in thread rolling.
Conventional thread rolling dies are typically made from high speed steels as well as other tool steels. Thread rolling dies made from steels have several limitations. The compressive strength of high speed steels and tool steels may not be significantly higher than the compressive strength of common workpiece materials such as alloy steels and other structural alloys. In fact, the compressive strength of conventional thread rolling die materials may be lower than the compressive strength of high strength workpiece materials such as, for example, nickel-base and titanium-base aerospace alloys and certain corrosion resistant alloys. In general, the compressive yield strength of tool steels used to make thread rolling dies falls bellow about 275,000 psi. When the compressive strength of the thread rolling die material does not substantially exceed the compressive strength of the workpiece material, the die is subject to excessive plastic deformation and premature failure.
In addition to having relatively high compressive strength, thread rolling die materials should possess substantially greater stiffness than the workpiece material. In general, however, the high speed steels and tool steels that are currently used in thread rolling dies do not possess stiffness that is higher than common workpiece materials. The stiffness (i.e., Young's Modulus) of these tool steels falls below about 32×106 psi. Thread rolling dies made from these high speed steels and tool steels may undergo excessive elastic deformation during the thread rolling process, making it difficult to hold close tolerances on the thread geometry.
In addition, thread rolling dies made from high speed steels and tool steels can be expected to exhibit only modestly higher wear resistance compared to many common workpiece materials. For example, the abrasion wear volume of certain tool steels used in thread rolling dies, measured as per ASTM G65-04, “Standard Test Method for Measuring Abrasion Using the Dry Sand/Rubber Wheel Apparatus”, is about 100 mm3. Therefore, die lifetime may be limited due to excessive wear.
Accordingly, there is a need for thread rolling dies made from materials that exhibit superior combinations of strength, particularly compressive strength, stiffness, and wear resistance compared to high speed and other tool steels conventionally used in thread rolling dies. Such materials would provide increased die service life and also may allow the dies to be used to produce threads on workpiece materials that cannot readily be processed using conventional dies.